The 2D–3D growth mode transition during the initial stages of growth of highly strained InGaAs on GaAs is used to obtain quantum‐sized dot structures. Transmission electron micrographs reveal that when the growth of In0.5Ga0.5As is interrupted exactly at the onset of this 2D–3D transition, dislocation‐free islands (dots) of the InGaAs result. Size distributions indicate that these dots are ∼300 A in diameter and remarkably uniform to within 10% of this average size. The areal dot densities can be varied between 109 and 1011 cm−2. The uniformity of the dot sizes is explained by a mechanism based on reduction in adatom attachment probabilities due to strain. We unambiguously demonstrate photoluminescence at ∼1.2 eV from these islands by comparing samples with and without dots. The luminescent intensities of the dots are greater than or equal to those of the underlying reference quantum wells.

Direct formation of quantum‐sized dots from uniform coherent islands of InGaAs on GaAs surfaces

It is shown that the ordinary semiclassical theory of the absorption of light by exciton states is not completely satisfactory (in contrast to the case of absorption due to interband transitions). A more complete theory is developed. It is shown that excitons are approximate bosons, and, in interaction with the electromagnetic field, the exciton field plays the role of the classical polarization field. The eigenstates of the system of crystal and radiation field are mixtures of photons and excitons. The ordinary one-quantum optical lifetime of an excitation is infinite. Absorption occurs only when "three-body" processes are introduced. The theory includes "local field" effects, leading to the Lorentz local field correction when it is applicable. A Smakula equation for the oscillator strength in terms of the integrated absorption constant is derived.

a Quantum-Mechanical Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals.

Instabilities in semiconductor heterostructure growth can be exploited for the self-organized formation of nanostructures, allowing for carrier confinement in all three spatial dimensions. Beside the description of various growth modes, the experimental characterization of structural properties, such as size and shape, chemical composition, and strain distribution is presented. The authors discuss the calculation of strain fields, which play an important role in the formation of such nanostructures and also influence their structural and optoelectronic properties. Several specific materials systems are surveyed together with important applications.

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Structural properties of self-organized semiconductor nanostructures

Three- and low-dimensional inorganic semiconductors

We report the direct deposition of strained InGaAs‐dot structures with a diameter of about 15 nm on GaAs surfaces by metalorganic chemical vapor deposition growth. High resolution scanning electron micrographs show highly uniform quantum‐sized dots formed by the Stranski–Krastanow growth mode. The sharp photoluminescence emission band of buried dot structures indicates efficient carrier capture and a homogeneous heterointerface. The average dot size and area dot density can be controlled accurately by growth temperature, and InGaAs deposition thickness, respectively.

Highly uniform InGaAs/GaAs quantum dots (∼15 nm) by metalorganic chemical vapor deposition

We directly synthesize InAs microclusters embedded in a single-crystal GaAs matrix by molecular-beam epitaxy. Fractional monolayers of InAs are deposited on terraced (001) GaAs surfaces and subsequently overgrown with GaAs. Growth conditions are adjusted in situ by reflection high-energy electron diffraction to those favoring step-flow nucleation of both Ga and In adatoms. The resulting microscopic structural configuration is studied by double-crystal x-ray diffractometry and high-resolution electron microscopy. These experiments reveal that InAs growth takes place in fact by nucleation of In adatoms on step edges. An array of isolated InAs clusters of subnanometer size (quantum dots) is thereby formed within the GaAs matrix. The interface of the InAs clusters is in registry with the surrounding GaAs matrix and is thus defect-free. Several spectroscopic techniques, such as transmission, cw photoluminescence, photoluminescence excitation, and picosecond photoluminescence, are applied to get insight into the optical properties of this system. We show that the optical response of excitons attached to the InAs dots is determined by the zero-dimensional symmetry of the system. This effect is most evident when comparing the spontaneous emission of InAs dots and InAs planes, which in either case results from the relaxation of excitons to the emitting state followed by their radiative recombination. The reduced translational symmetry causes a progressive release of wave-vector conservation, thus modifying the selection rules that uniquely determine the interaction of excitons with phonons (relaxation) and photons (recombination).

InAs quantum dots in a single-crystal GaAs matrix.

We examine the nature of the optical transitions in InAs/GaAs single-monolayer quantum wells grown by molecular-beam epitaxy. The heavy- and light-hole character of the observed transitions is revealed by monitoring the linear polarization of the emitted luminescence and by selective excitation of the states by linearly polarized light. In addition, these experiments provide information about the band alignment between InAs and GaAs.

H. Lage

Papers

InAs quantum dots in a single-crystal GaAs matrix.

Heavy- and light-hole character of optical transitions in InAs/GaAs single-monolayer quantum wells.

Role of broken translational invariance for the optical response of excitons in low-dimensional semiconductors